1. Field of the Invention
The present invention relates to an optical fiber for a fiber Bragg grating, and more particularly, to an optical fiber for use in an optical waveguide type Bragg grating, in which a diffraction grating is formed within the optical fiber or the optical waveguide, and a fiber Bragg grating using the same.
2. Description of the Related Art
There are various kinds of diffraction gratings for optical devices. However, in the case of a diffraction grating for use in an optical communications system, it is suitable to use an optical waveguide type diffraction grating which is easily connected to an optical waveguide and has a low insertion loss. A conventional optical waveguide type diffraction grating is fabricated by ultraviolet ray irradiation of a quartz-based optical fiber having a high-refractive index core doped with germanium oxide (GeO.sub.2) to cause a periodic change in the refractive index of the core.
However, in the diffraction grating obtained by the above conventional method, the pattern spacing of the refractive index changes is affected by thermal expansion. Also, since the effective refractive index changes according to temperature, the reflection wavelength (to be referred to as a "Bragg wavelength") changes with changes in the ambient temperature of the location at which the diffraction grating is installed.
Examples of optical fibers and optical fiber gratings of the conventional art are seen in the following U.S. Patents. U.S. Pat. No. 5,755,850, to Martin et al., entitled METHOD OF MAKING A SURGICAL LASER FIBER FROM A MONOLITHIC SILICA TITANIA GLASS ROD, and U.S. Pat. No. 5,829,445, to Martin et al, entitled METHODS FOR LASER TREATMENT OF TISSUE, describe an optical fiber using an ultra-low expansion material for use in surgical laser systems. The ultra-low expansion material consists of a silicon dioxide core doped with titanium dioxide which is cladded and jacketed for chemical and abrasion resistance. These patents do not deal, however, with aspects of optical fiber gratings.
U.S. Pat. No. 4,616,901, to MacChesney et al., entitled DOPED OPTICAL FIBER, discusses optical fibers in which the core is doped with nonglass forming refractory oxides including zirconia. The thermal expansion properties and use of such fibers in optical gratings are not discussed, however.
U.S. Pat. No. 5,868,734, to Soufiane et al., entitled METHODS OF USING SILICA-TITANIA CLAD FIBERS, discusses an optical fiber with a silica-titania glass cladding. The silica-titania glass provides a relatively low thermal expansion coefficient apparently providing protection for the fiber, but the patent does not discuss use in optical gratings.
U.S. Pat. No. 5,694,503, to Fleming et al., entitled ARTICLE COMPRISING A TEMPERATURE COMPENSATED OPTICAL FIBER REFRACTIVE INDEX GRATING, describes an optical fiber grating attached to a support member with a negative coefficient of expansion. The negative coefficient expansion substrate is external to the fiber, and is composed of a material such as zirconium tungstate. This invention thus requires device external to the fiber to achieve temperature compensation.
Based on our reading of the art, then, we have decided that what is needed is a Bragg optical fiber grating in which the Bragg wavelength is not seriously affected by temperature.